Method for improving the machinability of steel



UnitedStates Patent Elliot S. Nachtman, Park Forest, 111;, assignor toLa Salle Steel Co., Hammond, Ind., acorporation of Delaware No Drawing.A plication September 30, 1954, Serial No. 459,527

13 Claims. (Cl. 148-4) This invention relatesto-plain carbon and alloysteels and to a method of improving the machinability thereof.

It is concerned more particularly with a method of improving themachinability of plain carbon and alloy steels accompanied withimprovements also in physical and mechanical properties of the steel.

It is an object of this invention to produce and to provide a method forproducing plain carbon and alloy steels in which the machinability isimproved by means easily and simply carried out during normal steelmaking and it is a related object to improve the machinability of steelwith improvement also in other physical and mechanical properties of thesteel, such as strength, resistance to wear, resistance to corrosion andtoughness.

Numerous attempts have been made in the past to improve themachinability of such plain carbon and alloy steels. To the present,lead, selenium, sulphur and his muth have been found to providenoticeable improvements in the machinability of steel when added toprovide certain concentration in the steels but certain undesirableconditions have been associated with their use. For example, improvedrnachinability has been secured by the addition of lead in themanufacture of leaded steels but since lead is insoluble in the steel,it is diflicult to achieve the desirable uniform distribution of thelead inthe steel. In addition, lead releases toxic fumes under theprocessing conditions with the result that extensive precautions must betaken in the use thereof.

Similarly the distribution of sulphur in the desired concentration inthe steel is also difficult to control and unless combined withmanganese in substantial amounts, the sulphur will produce anundesirable condition in the steel during rolling, generally referred toas hot shortness. Bismuth and selenium have been used on a minor scalein stainless steel for the purpose of improving machinability. However,the additional cost occasioned by the use of such materials in desiredamounts is a deterrent to their adoption. In addition, these variouselements which have been added to steels to improve machinability inmany cases cause an undesirable loss in some of the mechanical andphysical properties of the steel.

It has been found in accordance with the invention described and claimedin my copending application 'Ser. No. 390,739, filed November 16, 1953,that the machinability of plain carbon and alloy steels can be improvedmaterially by the addition of lead and copper in combination in amountsranging from 0.03 to 0.35 percent by weight copper and 0.03 to 0.5precent .by weight lead. The improvements resulting from the combinationof copper and lead in such steels have been found to be enhanced furtherby the presence of sulphur, phosphorus and manganese.

It has now been found that copper alone can be used effectively inappropriate concentrations, without lead, to improve the machinabilityof plain carbon and alloy steels. It appears from analysis andobservations of results that copper-alone functions in a mannersubstantially ice unlike that of lead; Whereas lead is substantiallyinsoluble in steel, copper is soluble in steel, and whereaslead reducessome of the other properties ofsteel, copper provides additionalimprovements in the strength, corrosion resistance, and impact strengthof plain carbon and alloy steel. Unlike lead, the copper in the desiredconcentration may be added to steel by normal procedure, requiring nospecial techniques, with practically percent recovery in the steel and.with uniform distribution.

To the best of my knowledge, no one before has introduced copper inplain carbonand alloy steelsv intentionally for the improvement ofmachinability and such formulation to modify the chemistry of thesteelby'the presence of copper to improve machinability is believed torepresent a completely new concept in the field of metallurgy. Thediscovery that'copper alone is effective in improving the machinabilityof steel is believed to constitute an important advance in thetechnology of such steels, particularly since along with achievingimprove ment in machinability, desirable improvements in physical andmechanical properties are also secured. Such improvements have not beenobtainable with elements previously addedto modify the chemistry ofsteel to improve machinability.

Little, if any, improvement in machinability is secured whencopp'er ispresent in the plain carbon or alloy steel in amounts less than0.06percent by weight. Best results are secured and a marked improvement inmachinability as well as other physical and mechanical properties, suchas impact strength, corrosion resistance, and tensile strength aresecured when copper is present in the steel in amounts within the rangeof 0.15 to 0.30 percent by weight. To the present, improvements in themachinability of the steel'have been secured by the use of copper inamounts up to 0.60 percent by weight, but it will be understood that theimprovements in machinability can be securedby the use of. copper ingreater amounts.

Improvements in machinability of steel by the use of copper arenoticeable with low carbon steel (less than 0.25 percent carbon) as wellas-with medium carbon steel (0.25 to 0.50 percent carbon) and highcarbon steels (above 0.5 percent carbon). The general effect, forexample, is illustrated in Table I in which a steel containingapproximately 0.45 percent carbon was tested in a screw machine. Forcomparison, a steel of substantially the same composition was employedexcept for the presence of copper in one and the absence of copper inthe other. A measure of the relative machinability was obtained bymeasuring'the wear on'the tools after 90 of the parts being machined hadbeen cut under substantially identical con- In the test described inTable 1, parts were machined in a cycle time of 60 seconds, using a'surface speed of feet per minute and tool wear was measured after '90pieces had been cut. The difference in ToolfWear is an indication of thedifference in machinability of the two steels.

It will be apparent'from the datasecured that the steel containingcopper showed approximately '1 7 percent better wear on the inch turningtool; 64 percent better tool wear on the /2 inch turning tool, and 38percent better tool wear on the form tool.

The inventive concepts can be further illustrated by the effects ofcopper on the machinability of a low carbon steel (Table H).

1 This is a comparative machinability reading obtained by controlledmad'rtning on a constant pressure lathe in a standard test.(Transactions of the, ASME for July 1949 Constant-Pressure Lathe Testfor Measuring the Machinability of Free-Cutting Steels, by F. W Boulger,H. L. Shaw, and H. E. Johnson.) j

In the past it has been recognized that residual metals such as nickeland chromium have caused marked depreciation in the machinability ofsteels even when present in small amounts. Contrary to the acceptedlimitations, it has been found that the deleterious efiects of theresidual metals on machinability ofsteels is'reduced by the use ofcopper in accordance with the practice of this invention. While it ispreferred to make use of a steel in which such residual metals are notpresent since copper is capable then of maximum use, it has been foundthat the presence of residual metals does not destroy the machinabilitycharacteristic of the steel in the presence of copper even though lessimprovement is secured by copper, nevertheless a steel containing copperin combination with the residual metals provides for bettermachinability than the same steel without copper or within themachinability available from the copper is thusly efiected.

It has been found that the improvement in machinability available fromthe use of copper in plain carbon steels and alloy steels can beextended by the addition of other elements with the copper, such forexample as phosphorus, sulphur and nitrogen. To some extent these otherelements have been used before because of their beneficial eifects onmachinability but the improvement which is secured in a'system whichmakes use both of copper and sulphur together in plain carbon and alloysteels exceeds that which might be expected by way of aggregation to theextent that improvement in machinability is indicative more of asynergistic effect between these materials in steel. These unexpectedresults are secured when sulphur is present in amounts ranging from 0.01to 1.0 percent by weight in the steel and when the copper is present inan amount within the range of 0.06 to 0.6 percent by weight of thesteel.

When phosphorus is present, the best results are secured when theconcentration of phosphorus is within the range of 0.01 to 0.2 percentby weight, and the copper is present within an amount ranging from 0.06to 0.6 percent by weight, and sulphur, when present, is present in anamount less than 1.0 percent by weight. Nitrogen in amounts greater than0.001 percent by weight also improves the. machinability of steed whenpresent with copper alone or in a system which includes copper and oneor more other elements, such as sulphur, phosphorus and nitrogen.

The following steels of the non-austenitic type are representative ofthe chemistries of steel or other iron base alloys embodying thefeatures of this invention having out the residual metals and copper.improved machinability. In the examples the amounts Table III Change inSize Cycle Surface No. of

0 Mn S Nl Cr Cu Time, Speed Pieces Sec. PerMln. Cut %"Turn- Form ingT001 T001 .It will be apparent from the above table that the tool wearof medium carbon steels of similar composition are given are in percentby weight of the metal with the balance being substantially all iron.

C On Din P 81 N1 Mo 01 S N 15-. 23 00-. 00 1. 35-1. 05 04 17-. 22 05 051&0. 5

improved as much as 400 percent by the addition of cop- In themanufacture of steel and other iron base alloys, per and that thepresence of nickel and chromium recopper may be introduced into themetal when in the duces the improvement secured by the presence ofcopper, furnace, ladle or mold, in the form of elemental copper, yet thecopper present with such residual elements procopper oxygen sulphide,copper sulphide or as copper vides for animprovement in machinabilityover and above 70 sulphate or as a master metal containing copper, suchthe alloy without copper. In accordance with the inforas aniron-copper-sulphur alloy. It is an important conmation which has beendeveloped to the present, the residcept of this invention that by theaddition of copper in ual metals can be present in amounts totaling 0.25perthe forms described, normal operating procedures may cent by weightof the steel with a maximum of 0.05 5 be employedinthe metallurgicalprocesses for preparation percent by weight of any one element beforeimprovement of the steel. Recoveries of copper are essentially .100

percent and-such additions to incorporate copper can be made- Withouthazards and without requiring precautionary steps to be taken for thesafety of personnel or equipment.

An important concept of this invention resides in the further discoverythat the machinability of steel, for example as measured by the constantpressure lathe test reviously described, especially steels having copperpresent in the amounts described, show unexpectedly large improvementsin machinability when cold worked. As used herein, the term cold workingis meant to include deformation of the steel, as by cold drawing orrolling at room temperature or at elevated temperatures, such asdescribed in copending applications Ser. No. 293,431, now abandoned,Ser. No. 293,432, now abandomed, and Ser. No. 293,433, now abandoned.The most striking effects by way of improved machinability by coldWorking become apparent with steels containing 0.50 carbon or less.Particularly outstanding results are secured with steels having 0.15 to0.35 percent carbon. To the present, such improvements in machinabilityare maximized by cold working the steel for reduction in excess ofpercent in cross sectional area and generally with reductions of between-50 percent depending upon the chemistry of the metal, the size of theraw material being cold worked, and in the case of drawing at elevatedtemperatures, the temperature of drawing.

The unexpected improvement in machinability of steel by cold working isillustrated by the following table which compares a steel having copperwith the steel of the same composition except for copper. The two steelswere reduced by cold drawing bars at room temperature, cold drawing barswhich are at room temperature followed by strain relieving at hightemperature, and cold drawing bars which are at an elevated temperature.Raw material for the test was hot rolled bar stock 1% inches indiameter.

Table IV Composition Composition B Carbon Manganese Phosphorus SulphurSiliconr Copper Machinability index: 1

10% draft at room temperature followed by strain relieving 10% draft atelevated temperature 15% draft at room temperature 21% draft at roomtemperature 1 See footnote Table II.

it will be apparent from the data set forth in the above table that themachinability of steels containing copper within the amounts describedis unexpectedly greatly improve-d by cold reductions, for example as bymore than a 10 percent reduction in cross sectional area on a 1 inchbar, as compared to the machinability character istics of steel havingless than the described amounts of copper. When steel is drawn at aboutroom temperature to achieve the desired reduction, strain relieving byheat treatment at a temperature above 550 F. but below the lowercritical temperature and preferably within the range of 550-950" F. maybe desirable. W1th steels having from 0.2 to 0.4 percent carbon, maximumimprovement in machinability is secured by taking heavy drafts which maybe followed by strain relieving, especial- 1 when copper is present insuch steels alone or in a system with sulphur and with less than 0.15residual metals composed of nickel, chromium and molybdenum. Whenworking is achieved by drawing at' elevated temperatures as described inthe aforementioned copending applications, subsequent strain relieving.steps by heat treatment become unnecessary. V I

While cold working has been found tobe desirable to improve themachining. charcteristics of steel, the coinbination of cold workingsteels of thetype heretofore described cont'aining copper alone" orcontaining copper in a system with sulphur, and/or nitrogen, offersstill further possibilities for providing high levels of ma chinabilitytogether with improved strength, resistance to corrosion, toughness,wear and other mechanical and physical properties without introducinglimitations in the processing chracteristics of the steel. I

The unexpected improvements in machinability secured by metal working isnot fully understood. It has been found that steels, particularly thosecontaining carbon in the range of from 10-30 percent, can appreciably bebenefited by heavy cold working. These beneficial efiects with respectto machinability are enhanced when the steel contains copper alone or inthe presence of .phosphorus, sulphur, and/or nitrogen. Whatever thereason, the machin'ability of steels and iron base alloys, as determinedby the energy required for metal separation and by reduction in Wear ofthe turning and forming tools has been found greatly to be improved bymodific'ation in the chemistry of the steel to include copper as anessential component thereof, and further by working of the steel as bycold drawing to secure heavy drafts. Such improvements in machinabilityhave been instru mental in accelerating output of steel products byeuabling p'rccess'ing'with heavy feeds and higher speeds to increase theproduction rate while at the same time reducing the time required forreplacement and repair of tools and parts. Iii addition to increasedoutput at reduced costs, the presence of copper to improveinachinability has also provided im rovements in other physical andmechanical properties as previously pointed out.

Though not equivalent, it is suggested that improve nient inmachinability of such iron base" alloys and steel can be obtainedby theaddition'of cobalt, zin'c, cadmium, mercury, or tin. The chemicalcombination of these other elements in steel or other iron base alloys"appears to correspond more with that observe'd for copper as compared tothat from lead. The amounts required of these other elements aresomewhat similar to that for copper with variations depending upon thesolubility of the element in the iron or steel base alloy and theparticular chemistry of the steel.

It will be understood that changes may be made in the details offormulation, methods of incorporation and processing of the varioussteels prepared in a manner to provide the characteristics of thisinvention without departing from the spirit of the invention, especiallyas defined in the following claims.

I claim: I p

1. The metallurgical process'for improving the machim ability of steelcomprising the steps of advancing the steel through a die to effectreduction in cross-sectional area wherein the steel is a free machiningsteel of the nonaustenitic type having copper present as an essentialeler'nent in the range of 0.15 to 0.30 percent by weight, 0.04 to 0.50percent by weight sulphur, and lesstharl' 0.25 percent by weightresidual metals selected fr'o'ni' th'e'" group consisting of' nickel,chromium, vanadium and molyb= den'urn, with a maximum of 0.05 percent byweight of any one residual metal, and subsequently" machining the steelto produce parts. 7

2. The metallurgical process for improving the machinability of steelcomprising the steps of advancin the steel through a die to effectreduction in cross' sectional area wherein the steel is a free machiningsteel of "the non austenitic type having copper present as an essentialelementin the range of 0.15 'to'0.30 percent by weight, 0.04 to 0.50percentby weightsulphunand less than 0:25

percent by weight residual metals selected from the group consisting ofnickel, chromium, vanadium and molybdenum, with a maximumof 0.05 percentby weight of any one, strain relieving the steel, and subsequentlymachining the steel to produce parts.

3. The metallurgical process for improving the machinability of steelcomprising the steps of drawing the steel through a die to effectreduction in cross-sectional area wherein the steel is a free machiningsteel of the nonaustenitic type having copper present as an essentialelement in the range of 0.15 to 0.30 percent by weight, 0.04 to 0.50percent by weight sulphur, and less than 0.25 percent by weight residualmetals selected from the group consisting of nickel, chromium, vanadiumand molybdenum, with a maximum of about 0.05 percent by weight of anyone residual metal, and subsequently machining the steel to produceparts.

4. The metallurgical process for improving the machinability of steelcomprising the steps of drawing the steel through a die to eiiectreduction in cross-sectional area wherein the steel is a free machiningsteel of the non austenitic type having copper present as an essentialelement in the range of 0.15 to 0.30 percent by weight, 0.04 to 0.50percent by weight sulphur, and less than 0.25 percent by weight residualmetals selected from the group consisting of nickel, chromium, vanadiumand molybdenum, with a maximum of about 0.05 percent by Weight of anyone residual metal, strain relieving the steel, and subsequentlymachining the steel to produce parts.

5. The metallurgical process for improving the machinability of steelcomprising the steps of extruding the steel through a die to effectreduction in cross-sectional area wherein the steel is a free machiningsteel of the nonaustenitic type having copper and sulphur present asessential elements in the range of 0.15 to 0.30 percent by weight copperand 0.04 to 0.50 percent by weight sulphur, and less than 0.25 percentby weight residual metals selected from the group consisting of nickel,chromium, vanadium and molybdenum, with a maximum of 0.05 percent byweight of any one residual metal, and subsequently machining the steelto form parts.

6. The metallurgical process for improving the machinability of steelcomprising the steps of advancing the steel through a die to effectreduction in cross-sectional area wherein the steel is a free machiningsteel of the nonaustenitic type having copper present as an essentialelement in the range of 0.15 to 0.30 percent by weight, 0.04 to 0.50percent by weight sulphur, 0.01 to 0.20 percent by weight phosphorus,and less than 0.25 percent by weight residual metals selected from thegroup consisting of nickel, chromium, vanadium and molybdenum, with amaximum of 0.05 percent by weight of any one, and subsequently machiningthe steel to form parts.

7. The metallurgical process for improving the machinability of steelcomprising the steps of advancing the steel through a die to effectreduction in cross-sectional areawherein the steel is a free machiningsteel of the non-austenitic type having copper present as an essentialelement in the rangeof 0.15 to 0.30 percent by weight, 0.04 to 0.50percent by weight sulphur, 0.01 to 0.20 percent by weight phosphorus,more than 0.001 percent by weight nitrogen, and less than 0.25 percentby weight residual metals-selected from the group consisting of nickel,chromium, vanadium and molybdenum, with a maximum of 0.05 percent byWeight of any one, and subsequently machining the steel to form parts.

8. The metallurgical process for improving the machinability of steelcomprising the steps of advancing the steel through a'die to efiectreduction in cross-sectional area wherein-the steel is a free machiningsteel of the non-'austenitic type havingcopper present as an essentialelement in the range of 0.15 to 0.30 percent by weight,

0.04to 0.50 percent by weight sulphur, 0.01 to 0.20 percent by weightphosphorus, more than 0.001 percent by weight nitrogen, andless than0.25 percent 'by weight residual metals selected from the groupconsisting of nickel, chromium, vanadium and molybdenum, with a maximumof 0.05 percent by weight of any one, strain relieving the steel, andsubsequently machining the steel to form parts.

9. The metallurgical process for improving the machinability of steelcomprising the steps of advancing the steel through a die to eifectreduction in cross-sectional area wherein the steel is a low carbon freemachining steel of the non-austenitic type having up to 0.5 percent byweight carbon, copper present as an essential element in an amountwithin the range of 0.15 to 0.30 percent by weight, 0.04 to 0.50 percentby weight sulphur, and less than 0.25 percent by weight residual metalsselected from the group consisting of nickel, chromium, vanadium andmolybdenum with a maximum of 0.05 percent by weight of any one, andsubsequently machining the steel to form parts.

10. The metallurgical process for improving the machinability of steelcomprising the steps of heating the steel to a temperature within therange of 450 F. to the lower critical temperature for the steelcomposition, advancing the heated steel through a die to effect reduction in cross-sectional area while the steel is at a temperature withinthe range of 450 F. to the lower critical temperature for the steelcomposition and wherein the steel is a free machining steel of thenon-austenitic type having copper present as an essential element in anamount within the range of 0.15 to 0.30 percent by weight, 0.04 to 0.50percent by weight sulphur, and less than 0.25 percent by weight residualmetals selected from the group consisting of nickel, chromium, vanadiumand molybdenum with a maximum of about 0.05 percent by weight ofany one,and subsequently machining the steel to form parts.

11. The metallurgical process for improving the machinability of steelcomprising the steps of heating the steel to a temperature within therange of 450 F. to the lower critical temperature for the steelcomposition, advancing the heated steel through a die to effectreduction in cross-sectional area while the steel is at a temperaturewithin the range of 450 F. to the lower critical tempera ture for thesteel composition and wherein the steel is a free machining steel of thenonaustenitic type having copper present as an essential element in anamount within the range of 0.15 to 0.30 percent by Weight, 0.04 to 0.50percent by weight sulphur, and less than 0.25 percent by weight residualmetals selected from the group consisting of nickel, chromium, vanadiumand molybdenum with a maximum of 0.05 percent by weight of any one,strain relieving the steel, and subsequently machining the steel to formparts.

12. The metallurgical process for improving the machinability of steelcomprising the steps of advancing the steel through a draw die to efiectreduction in crosssectional area While the steel is at a temperaturewithin the range of 450 F. to the lower critical temperature for thesteel composition and wherein the steel is a free machining steel of thenon-austenitic type having up to 0.50 percent by weight carbon, 0.15 to0.30 percent by weight copper, 0.04 to 0.50 percent by weight sulphur,0.1 to 0.20 percent by weight phosphoius, more than 0.001 percent byweight nitrogen, and less than 0.25 percent by weight residual metalsselected from the group consisting of nickel, chromium, vanadium andmolybdenum with a maximum of 0.05 percent by weight of any one, andsubsequently machining the steel to form parts.

13. The metallurgical process for improving the machinability of steelcomprising the steps of advancing the steel through a draw die to effectreduction in cross-sectional area while the steel is at a temperaturewithin the range of 450 F. to the lower critical temperature for thesteel composition and wherein the steel is a free machining steel of thenon-austenitic type having up to 0.50 percent by weight carbon, 0.15 to0.30 percent by weight copper, 0.04 to 0.50 percent by weight sulphur,0.01 to 0.20 percent by weight phosphorus, more than 0.001 percent byweight nitrogen, and less than 0.25 percent by 5 weight residual metalsselected from the group consisting of nickel, chromium, vanadium andmolybdenum with a maximum of 0.05 percent by weight of any one, heatingto an elevated temperature to strain relieve the steel, and subsequentlymachining the steel to form parts.

References Cited in the file of this patent UNITED STATES PATENTS1,957,427 Buckholtz May 8, 1934 OTHER REFERENCES 10 Metals Handbook,1948 ed., pp. 309, 310, 369, 370,

Open Hearth Proceedings," pub. by AIMME, vol. 38,

1. THE METALLURGICAL PROCESS FOR IMPROVING THE MACHINABILITY OF STEELCOMPRISING THE STEPS OF ADVANCING THE STEEL THROUGH A DIE TO EFFECTREDUCTIN IN CROSS-SECTIONAL AREA WHEREIN THE STEEL IS A FREE MACHININGSTEEL OF THE NONAUSTENITIC TYPE HAVING COPPER PRESENT AS AN ESSENTIALELEMENT IN THE RANGE OF 0.15 TO 0.30 PERCENT BY WEIGHT, 0.04 TO 0.05PERCENT BY WEIGHT SULPHUR, AND LESS THAN 0.25 PERCENT BY WEIGHT RESIDUALMETALS SELECTED FROM THE GROUP CONSISTING OF NICKEL, CHROMIUM, VANADIUMAND MOLYBDENUM, WITH A MAXIMUM OF 0.05 PERCENT BY WEIGHT OF ANY ONERESIDUAL METAL, AND SUBSEQUENTLY MACHINING THE STEEL TO PRODUCE PARTS.